DNA strand breaks are induced by ionizing radiation and mutagenic chemicals, and they also arise during replication of a damaged DNA template. If not handled properly, these strand breaks often lead to gross chromosome rearrangements. Homologous recombination is a major means for DNA strand break repair in eukaryotes. Importantly, there is compelling evidence that homology-directed DNA repair is needed for cancer avoidance in humans. In the homologous repair of a double-stranded break, the ends of the break are subjected to exonucleolytic processing to yield 3' single-stranded DNA tails a few hundred bases in length. Nucleation of various recombination factors onto these DNA tails renders them recombinogenic, leading to a search for the chromosomal homolog and the stable pairing of the DNA tails with the homolog to form a DNA joint called "D-loop". In the later stages, DNA synthesis occurs, followed by the resolution of DNA intermediates to yield mature recombinants and restore the integrity of the injured chromosome. Extensive genetic evidence indicates that the evolutionarily conserved genes of the RAD52 epistasis group, of which RAD54 and RDH54 are key members, mediate homologous recombination and DNA strand break repair by recombination. Rad54 and Rdh54 proteins have been purified to near homogeneity from yeast cells and found to possess DNA-dependent ATPase and ATP-hydrolysis-driven DNA supercoiling activities. Both Rad54 and Rdh54 dramatically stimulate the Rad51/RPA-mediated D-loop reaction. Further studies are proposed to delineate the action mechanism of Rad54 and Rdh54 proteins in DNA supercoiling and heteroduplex DNA formation, and also to define the role of Rad54 and Rdh54 in chromatin remodeling. In addition, we will examine the functional and physical interactions between Rdh54 and the Rad50-Mrel 1-Xrs2 complex germane for understanding the break-induced replication mode of recombination. The studies should provide insights about the mechanisms of the major recombination pathways in eukaryotic cells.